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You should look at Eclipse. It has a concept of help that shows topics for the plug-ins you currently have loaded. Eclipse is an interesting product... The bare-bones core is essentially a server that manages feature modules. One of those modules is the Help system. At least that's how it was in 2006 or so when I last looked at using Eclipse as a help server. (Yes, at least back then you could install a stand-alone Eclipse that only served Help... And it's an open license.) The theory is that you could exploit the Eclipse capability to show/hide "plug-in" docs... Find the trigger that tells it to recognize this or that module of documentation. Whether this could seamlessly "blur" the topics into a continuous hierarchy, I don't know. (I never got beyond the presentation to my department... Doc departments are known for their inertia.)
The time is fast approaching when this capability will be a requirement. Products are leaving the desktop and emigrating to variable constellations of virtual servers. Shoot, even the desktop can be virtualized. Fasten your seat belts...
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